Polyurethane foams are widely known and used in automotive, housing and other industries. Such foams are produced by reaction of a polyisocyanate with a polyol in the presence of various additives. One such additive is a chlorofluorocarbon (CFC) blowing agent which vaporizes as a result of the reaction exotherm, causing the polymerizing mass to form a foam. The discovery that CFC's deplete ozone in the stratosphere has resulted in mandates diminishing CFC use. Production of water-blown foams, in which blowing is performed with CO.sub.2 generated by the reaction of water with the polyisocyanate, has therefore become increasingly important. Tertiary amine catalysts are typically used to accelerate blowing (reaction of water with isocyanate to generate CO.sub.2) and gelling (reaction of polyol with isocyanate).
The ability of the tertiary amine catalyst to selectively promote either blowing or gelling is an important consideration in selecting a catalyst for the production of a particular polyurethane foam. If a catalyst promotes the blowing reaction too selectively, much of the CO.sub.2 will be evolved before sufficient reaction of isocyanate with polyol has occurred, and the CO.sub.2 will bubble out of the formulation, resulting in collapse of the foam. A foam of poor quality will be produced. In contrast, if a catalyst too strongly promotes the gelling reaction, a substantial portion of the CO.sub.2 will be evolved after a significant degree of polymerization has occurred. Again, a poor quality foam, this time characterized by high density, broken or poorly defined cells, or other undesirable features, will be produced.
Tertiary amine catalysts generally are malodorous and offensive and many have high volatility due to low molecular weight. Release of tertiary amines during foam processing may present significant safety and toxicity problems, and release of residual amines from consumer products is generally undesirable.
Amine catalysts which contain active hydrogen functionality (e.g., --OH, --NH.sub.2, and --NHR) often have limited volatility and low odor when compared to related structures which lack this functionality. Furthermore, catalysts which contain active hydrogen functionality chemically bond into the urethane during the reaction and are not released from the finished product. Catalyst structures which embody this concept are typically of low to moderate activity and promote both the blowing (water-isocyanate) and the gelling (polyol-isocyanate) reactions to varying extents.
Secondary alcohols are preferred in the structures because these catalysts exhibit a desirable balance between their promotion of the active hydrogen-isocyanate reactions and their own reactivity with isocyanates. In contrast, catalysts which contain primary alcohols react rapidly with isocyanates and thus high use levels are required. Catalysts which contain tertiary alcohols react slowly with isocyanates, but the urethanes of tertiary alcohols which are formed have poor thermal stability. These urethanes may degrade and release the catalyst at temperatures substantially below the decomposition temperature of the foam itself. The free amine could then accelerate foam decomposition.
Catalysts containing active hydrogen functionality of the type --NH.sub.2 and --NHR are less well known.
Catalysts which strongly promote the water-isocyanate (blowing) reaction include tertiary amine structures based on the diethylenetriamine skeleton, in particular pentamethyidiethylenetriamine, and the .beta.-(N,N-dimethylamino)alkyl ethers, in particular bis(N,N-dimethylaminoethyl) ether. Low odor, reactive catalysts structurally related to bis(N,N-dimethylaminoethyl) ether are described in U.S. Pat. No. 4,338,408 and 4,433,170. Texacat.RTM. ZF-10 catalyst, 2-[N-(dimethylaminoethoxyethyl)-N-methylamino]ethanol, is an effective blowing catalyst, albeit less effective than bis(N,N-dimethylaminoethyl) ether.